In recent years, devices using large capacity wireless communication such as a cellular phone are spreading. In such wireless communication, a predetermined band is divided into a plurality of bands and communication is performed in each band to increase communication capacity. However, when antennas corresponding to each of a plurality of bands are used, a communications device or communication terminal becomes larger. Thus, an antenna capable of corresponding to a plurality of bands has been proposed.
For example, a multi-band antenna in which a plurality of antennas corresponding to each of a plurality of bands are disposed and a plurality of antennas are selectively used without using an antenna duplexer (Patent Literature 1). In this antenna, a feeding end of one feeding line and a plurality of antenna units having different resonant frequencies are directly connected by micro strip lines. Then, by providing the micro strip lines with filter functions, the antenna units and the filters are integrally formed. An impedance of the micro strip line can be set by adjusting a line length of the micro strip line and by providing the micro strip line with a plurality of stubs.
Further, an antenna device that can easily adjust a plurality of resonant frequencies has been proposed (Patent Literature 2). The antenna device includes a T-shape element and a stub. The T-shape element includes a first end serving as a feeding point. The T-shape element is branched at the midpoint to include a second end and a third end. One end of the stub is connected between the midpoint and the second end and the other end is grounded. The stub and the T-shape element form a it-shape. Then, a first line length from the first end to the second end is longer than a second line length from the first end to the third end. The first line length and the second line length are lengths corresponding to a first resonant frequency and a second resonant frequency, respectively. Accordingly, this antenna can correspond to two signals of the first resonant frequency and the second resonant frequency.
Furthermore, an antenna element that can improve radiation efficiency regardless of a shape of a circuit board conductor and achieve a wider bandwidth has been proposed (Patent Literature 3). This antenna element includes a λ/2 radiation electrode disposed on a base surface made of dielectric material. The λ/2 radiation electrode extends from the center of the base surface in directions symmetrical to each other. A ground conductor line is connected to the vicinity of the center of the λ/2 radiation electrode, and a feeding conductor line is connected to a position at a predetermined distance from the center part of the λ/2 radiation electrode. At least one of both ends of the λ/2 radiation electrode is an open end. According to this configuration, it is possible to achieve impedance matching between the radiation electrode and a high-frequency signal source by adjusting a layout of the feeding conductor line and the ground conductor line. Therefore, it is possible to achieve the wider bandwidth.